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Lumpy idle,induction leak

ltn6a

Active Member
Hi to the great and the good of the forum

I have an IO- 320B1A in my RV 6A which has been suffering from a lumpy idle when hot , to cut a long story short the injector body ,distribution valve and nozzles have all been bench tested and are good , the mags have been bench tested and are also good . We got the engine running pretty good but the idle mixture adjustment wheel was almost at full travel in the rich direction .
After adding some additional heat shielding to the fuel system I then disconnected the mixture link and cantered the thumb wheel but keeping the overall length .
So now it all goes wrong on starting the engine pops and bangs and will not idle fuel flow and manifold pressure about 2gal hr and manifold press a bit 13 “
After adjusting the mixture we had more popping and banging and at some point during this fuel flow jumped to 6 gal hr and manifold press 22”
As I understand it this is indicative of an induction leak , initial visual check of the induction tubes ans connector sleeves all look ok .
Plan is to check for induction leak using the compression tester method and soapy bubbles .
Is there anything else I can check or something that may have produced these symptoms

Thanks in advance for any assistance

Mark
 
Make sure you are getting good airflow through your air filter. I saw a case recently where the owner installed a lawn mower filter (had no filter previously) and saw similar issues.

Look up the induction system smoke test on YouTube, as well.
 
Induction leaks are at their worst at idle when the differential between ambient atmospheric pressure and the induction systems internal pressure are greatest.

They leak hardly at all during full throttle operation; because the differential is nearly nonexistent at wide open throttle.

One check you might do is a mixture cutoff test. While at idle, with a warm engine; as you slowly pull the mixture, you should see a gain of at least 50 to 100 RPM before cutoff. Less than 50 you are too lean, more than 100 too rich.
 
Hi to the great and the good of the forum

I have an IO- 320B1A in my RV 6A which has been suffering from a lumpy idle when hot ,
Mark

How does it run when cold? Cold engines need more fuel per a given air flow than a warm engine and servos don't account for this at idle. Leaks in the induction system, with FI, will result in leaner fuel mixtures. WIth an induction leak, idle performance should be worse when cold than hot.

if this is only happening when hot/heat soaked, I would continue to work on getting the fuel cooler on it's way to the servo.

Larry
 
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Hi

Up until we started to get the induction leak after the popping and banging which I think was caused when I cantered the mixture thumb wheel the engine run well and smooth when cold and in flight the only issue was after heat soak when taxing back at low idle.
Due to do induction leak test using the compression tester tomorrow so will report back after that

Thanks all

Mark
 
Hi

Due to do induction leak test using the compression tester tomorrow so will report back after that

Thanks all

Mark

I'd like to see a detailed description of how this works. I get hung up on the idea that air will just flow out your air intake and you will be challenged to pressurize the induction system enough to show a leak.
 
If you have a multi-cylinder EGT gauge you can quickly check for an induction leak. The EGT's will RISE when the throttle is pulled to idle, rather than go lower. Very easy to check everytime you are in the pattern for a landing.
Pull the throttle to idle while on base and watch EGT's. Works every time. :)

Vic
 
Hi

...the engine run well and smooth when cold and in flight the only issue was after heat soak when taxing back at low idle.

Mark

That is not uncommon behavior for Lyc FI. Not what I would call classic induction leak behavior.

Larry
 
Does anyone have a source for silicone gaskets as an alternative to the paper ones that are used at the cylinder? Seems a rubber one would seal up irregularities a bit better than the thin paper ones.
 
The SDS No Leak is the ultimate solution to this problem.

It looks like it but my main issue is really just the O-rings at the airbox but I figured I would replace the gaskets anyway. My tubes fit directly into the airbox and do not have rubber hoses so that SDS system looks like a very expensive alternative to a few dollars worth of gaskets in my current situation.
 
Does anyone have a source for silicone gaskets as an alternative to the paper ones that are used at the cylinder? Seems a rubber one would seal up irregularities a bit better than the thin paper ones.

Been there, done that... Does not work in the long term. Also done dead soft copper, and "no blow" exhaust gaskets with less than stellar results.
 
While the SDS solution is really nice, I looked at that as a bit over-engineered for what's needed here. Was looking at McMaster a while ago and found they have a gasket making service. While it's somewhat pricey at $150 for 20 gaskets, they do have a nice material available which is perfect for this application which is made from graphite sandwiched with stainless layers to provide integrity. It's 1/16" thick, or about double the stock paper ones. I had a set made last month and installed and so far they're perfect. Obviously time will tell, but this looks to be a much better solution than the stock 'paper'. PM me for the dxf file. Also attached is the datasheet of this material.

20201014_064513.jpgView attachment SGL-Datasheet-SIGRAFLEX-HOCHDRUCK-EN.pdf
 
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Wait until you have a few hours on this new gasket before you deem the SDS solution "over engineered". Any flat gasket relies on the flawed concept of the induction tube standing proud of the flange retainer surface to seal. Over torque it just once and it bows the retainer and clamping force is lost. The SDS concept relies on the far more consistent compression of an O ring for sealing - you can't bow the retainer no matter how much you abuse the mount bolts. And no matter how tight the retainer, the O ring allows a significant articulation of the induction tube while maintaining seal.

There is a lot more going on with this joint than meets the eye.
 
But for intake tubes without the hose connection method one would have to cut and reweld the tubes or have new ones fabricated to get the collars switched out. I am not disputing the SDS as a better design but it still seems an improvement could be made with a thicker gasket that would take up the gap you mentioned resulting from over torque.
 
The issue with the Lycoming scheme is that the only sealing surface is the small flange around the tube itself. Its a very small amount of area, and yes, you need some level of compliance there to make up for surface irregularities and tube misalignment. The vast remainder of the gasket supports the tube retainer, and there is no sealing function there. Here you want a non-compliant, rigid interface. The requirements are in oposition, so finding the "right" balance of material properties is tough. The ideal "gasket" would be made of aluminum (for retainer stability) with an O ring bonded to it (for seal compliance). I deal with this type of seal with turbine aircraft so I know the concept is valid, but the cost is exceptionally high even for a mass produced item. Your new material may strike a better balance than the composition style generally available - please keep us informed.

That said, the initial cost for the SDS solution is certainly higher than a composition gasket, but so is the reliability/durability. And if you DO cut an O ring, the replacement is only pennies and takes a fraction of the time to replace compared to the paper gasket. How much is your time worth? After fighting dried gaskets and bowed retainers for the last 30 years, I'm happy to pay the modest cost of the SDS solution and being done with it once and for all.

Everyone has to do their own cost/benefit analysis and come up with a value proposition - but make sure to include more than initial cost.
 
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While the SDS solution is really nice, I looked at that as a bit over-engineered for what's needed here. Was looking at McMaster a while ago and found they have a gasket making service. While it's somewhat pricey at $150 for 20 gaskets, they do have a nice material available which is perfect for this application which is made from graphite sandwiched with stainless layers to provide integrity. It's 1/16" thick, or about double the stock paper ones. I had a set made last month and installed and so far they're perfect. Obviously time will tell, but this looks to be a much better solution than the stock 'paper'. PM me for the dxf file. Also attached is the datasheet of this material.

View attachment 4014View attachment 4017

What is the thickness?

Larry
 
Try removing and plugging the "overprimne" or "sniffle valve" from the bottom of the pan. Some cycle open and closed, causing erratic idle.

Regards,

Gary
 
Induction leak?

Reviewing my after flight data, and I see this in my EGT graphs, and wondering if this indicates an induction leak?

HB-YMM Possible induction leak graph 20201108.png

BTW, I have Ross's induction kit ordered, and it should be here "soon".
 
Lumby Idle, induction leak

I've got the same problem on my Superior IO320. During taxi for take off idle is steady. After landing engine surges about 100 to 150 RPMs on taxi. The SDS product will not work for me without major mods because I have horizontal cold induction. Intake and filter are good. Any suggestions on a fix?
 
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